This invention relates generally to the field of air moving apparatus such as fans and blowers. More specifically, the invention relates to an impeller for use in fans of the transverse type. Transverse fans are also known as cross-flow or tangential fans.
The operating characteristics and physical configuration of transverse fans make them particularly suitable for use in a variety of air moving applications. Their use is widespread in air conditioning and ventilation apparatus. Because such apparatus almost always operates in or near occupied areas, a significant design and manufacturing objective is quiet operation.
FIG. 1 shows schematically the general arrangement and air flow path in a typical transverse fan installation. FIG. 2 shows the main features of a typical transverse fan impeller. Fan assembly 10 comprises enclosure 11 in which is located impeller 30. Impeller 30 is generally cylindrical and has a plurality of blades 31 disposed axially along its outer surface. As impeller 30 rotates, it causes air to flow from enclosure inlet 21 through inlet plenum 22, through impeller 30, through outlet plenum 23 and out via enclosure outlet 24. Rear or guide wall 15 and vortex wall 14 each form parts of both inlet and outlet plena 22 and 23. The general principles of operation of a transverse fan are well known and need not be elaborated upon except as necessary to an understanding of the present invention.
When a transverse fan is operating, it generates a certain amount of noise. One significant component of the total noise output of the fan is a tone having a frequency related to the rotational speed of the fan multiplied by the number of fan blades (the blade rate tone). The passage of the blades past the vortex wall produces this blade rate tone. Discrete frequency noise is in general more irritating to a listener than broad band noise of the same intensity. The blade rate tone produced by the typical prior art transverse fan has limited the use of such fans in applications where quiet operation is required.
At least one prior art disclosure has proposed a means of reducing the blade rate tonal noise produced by a transverse fan. U.S. Pat. No. 4,538,963 (issued Sep. 3, 1985 to Sugio et al.) discloses a transverse fan impeller in which the circumferential blade spacing (called pitch angle in the patent) is random. Random blade spacing can be effective in reducing noise but can lead to problems in static and dynamic balance and to difficulties in manufacturing.
Blade rate tonal noise is not limited to fans of the transverse type. R. C. Mellin & G. Sovran, Controlling the Tonal Characteristics of the Aerodynamic Noise Generated by Fan Rotors, Am. Soc'y of Mechanical Eng'rs Paper No. 69 WA FE-23 (1969) (Mellin & Sovran) discusses the blade rate tonal noise associated with axial flow or propeller type fans and provides a technique for designing such a fan with unequal blade spacing so as to minimize blade rate tonal noise. Mellin & Sovran addresses axial fans only. Further, the authors wrote that their technique is limited to isolated rotors and that placing a body either upstream or downstream of the rotor would lead to acoustic interactions and the production of tones other than the blade rate tone. Not only does Mellin & Sovran not teach or suggest that its technique could be applied to fans of other than the axial flow type, it suggests that the presence of a body such as the vortex wall in a transverse fan installation would lead to interactions and production of tones such as to make questionable the application of the Mellin & Sovran technique to a transverse fan.
Further, at least one axial flow fan variant constructed according to the teaching of Mellin & Sovran will not be in balance, as the authors of the paper admit.
And Mellin & Sovran teaches that an axial flow fan with blades spaced by its method will have a reduced level of blade rate frequency noise, but that the overall noise level is approximately the same in comparison to a similar fan with equally spaced blades.